Biology Reference
In-Depth Information
Apocynaceae (S.J. Darbyshire, personal communication). In Europe there is evi-
dence of successful hybridization between PSW and another relative, white swal-
low-wort (
V. hirundinaria
Medik.) (Lauvanger and Borgen 1998). This evidence not
only contributes to taxonomic confusion regarding identification of swallow-worts,
but also points to the potential for pale and black wallow-worts to hybridize
(DiTommaso et al. 2005b).
There remains a great deal of confusion as to the correct taxonomic placement of
the invasive swallow-wort species, with some taxonomists placing them under the
genus
Cynanchum
and others in the genus
Vincetoxicum
. Given that there are a number
of native North American plants in the genus
Cynanchum
[22 according to the USDA
Plant Database (2008)], DiTommaso et al. (2005b) proposed that
Vincetoxicum
should
be used solely for the alien species in order to denote their old world origins. This view
is shared by Liede (1996), who, primarily on the basis of the presence of unique alka-
loids and glycosides, separates
Vincetoxicum
from
Cynanchum
and places PSWs and
BSWs within the genus
Vincetoxicum
(also Liede and Tauber 2002).
From a review of the literature, no studies have been performed to determine the
ploidy level or chromosome numbers for swallow-wort species or populations in
the USA, and other evaluations that have been performed were limited in their geo-
graphic focus. A chromosome count of 2
n
= 22 was reported for PSW plants in
Ottawa, ON, Canada (Moore 1959), while chromosome counts for BSW plants
vary from
n
= 11 in Spain (Diosdado et al. 1993) to 2
n
= 22 and 2
n
= 44 for two
populations in Italy (Aparicio and Silvestre 1985; Moore 1959). At this time, we
have no information on chromosome numbers of swallow-wort populations in the
USA, which could provide valuable information to more fully describe the geno-
typic relationships among and between these two species.
PSW is native to eastern regions of the Ukraine and southwestern portions of
Russia north of the Black Sea and Caucasus; BSW (
V. nigrum
) is endemic to south-
western Europe, particularly regions of the Iberian Peninsula, southern France, and
northern Italy (DiTommaso et al. 2005b). In their native ranges, the two species are
relatively rare and do not overlap, with scattered patches of 3-15 stems of BSWs
found to be typical of native populations in southern France (DiTommaso et al.
2005b; L.R. Milbrath, personal communication; Tewksbury et al. 2002). While
PSW has been reported as invasive in one case in Norway, the third
Vincetoxicum
species,
V. hirundinaria
, is actually much more widespread and has greater invasive
potential in Europe than the two species that are problematic in North America
(Lauvanger and Borgen 1998).
13.3
Reproductive Biology and Phenology
Flowering can begin as early as mid to late May for PSW populations in Central
New York and will peak several weeks later in early to mid June (Sheeley 1992).
Floral development can be delayed by 10 days in populations farther north in
New York, and by up to 4 weeks for populations in Ontario and other regions of
